Green’s Dictionary of Slang

horsefuck v.

[SE horse + fuck v. (1)]

to have sexual intercourse with the man using rear entry.

[US]J. Wambaugh Blue Knight 114: Sexy little twist [...] a good body. I’d like to break her open like a shotgun and horsefuck her.
[US](con. 1949) G. Pelecanos Big Blowdown (1999) 272: You ever horsefuck a fat girl, Joe?